Electromagnetic sensors are often used in downhole operations, including while drilling, to measure the physical properties of subsurface formations. Many of these electromagnetic sensors use one or more coils to generate magnetic fields. These sensors, for example, include so-called induction, propagation, and nuclear-magnetic resonance (NMR) tools, which may use one or more coils that operate as antennas to generate and/or receive magnetic fields. Often, in order to achieve the desired performance, these tools carry groups of coils with their magnetic vectors oriented in different directions. One problem with these conventional tools is that their bodies are often made from conductive material (e.g., metal), which makes it difficult for the coils to be positioned to perform in an optimal fashion. Another problem with these conventional tools is that the coils are different sizes requiring a separate calibration for the size and position of each coil. This makes it difficult to accurately tune and calibrate the coil system as a whole.
Thus, what are needed are electromagnetic sensors with improved coil performance. What are also needed are electromagnetic sensors with improved performance when the coils are located on conductive bodies. What are also needed are co-located antennas for use on downhole tools that are easier to tune and calibrate.